1. Field of the Invention
The present invention relates to a light-emitting device for compensating reduction in luminance of the light-emitting device.
2. Description of the Related Art
In recent years, wide-viewing-angle, high-moving-image-characteristics light-emitting devices using electroluminescence elements (EL elements) have been developed. The EL elements, which are self-emission elements that themselves emit light, have a problem in that the luminance is likely to decrease over time. When EL elements deteriorates, resistance thereof generally increases. Therefore, under a condition in which a certain voltage is applied to an EL element, the amount of current which flows into the EL element decreases as the EL element deteriorates, which cause reduction in the luminance.
In FIG. 1, a driving transistor 101 and an EL element 102 which is electrically connected to the driving transistor 101 are shown. A gate of the driving transistor 101 is electrically connected to a signal terminal 103; one of a source and a drain of the driving transistor 101 is electrically connected to a first power source terminal 104; the other of the source and the drain of the driving transistor 101 is electrically connected to one electrode (one of an anode and a cathode) of the EL element 102; and the other electrode (the other of the anode and the cathode) of the EL element 102 is electrically connected to a second power source terminal 105. High potential (or low potential) of a power source is applied to the first power source terminal 104, and low potential (or high potential) of the power source is applied to the second power source terminal 105. The driving transistor 101 controls the current which flows into the EL element 102 to determine the luminance of the EL element 102. Description hereinbelow is made on the case where the driving transistor 101 is a p-channel transistor, but the present invention is not limited to this case of the p-channel transistor. The driving transistor 101 may be an n-channel transistor.
A curve 201 in FIG. 2A shows an Ids (source-drain current) vs. Vds (source-drain voltage) characteristic of a p-channel transistor. The curve 201 includes a saturation region 202 and a linear region 203. When Vds is equal to a value obtained by subtracting the threshold voltage (Vth) of the transistor from the gate-source voltage (Vgs), that is, Vds=Vgs−Vth (this Vds is called a saturation drain voltage), Vds delimits a boundary between the linear region 203 and the saturation region 202. A dashed line 200 in FIG. 2A is a curve formed by a point which delimits the boundary between the linear region and the saturation region in the curve 201 when Vgs is changed. In other words, an intersection of the dashed line 200 and the curve which shows the Ids vs. Vds characteristic is a boundary point between the linear region and the saturation region.
In the case where the EL element 102 is driven with analog gray-scale displays, the driving transistor 101 is operated in the saturation region 202 in many cases. This is because, in the linear region, dependence on the gate voltage of the drain current is small, so that it is difficult to change the drain current significantly and it is not easy to drive the EL element with multiple gray-scale displays. Furthermore, this is because the dependence of Ids on Vds in the linear region 203 is larger than that in the saturation region 202. Description will be made on this below.
In FIG. 2B, a current-voltage curve (I-V curve) 204 of the EL element 102 is shown in addition to the transistor characteristic shown in FIG. 2A. Reference symbol Vcom denotes a reference potential of the EL element 102. An intersection 206 of the curve 204 and the curve 201 corresponds to an operation point of the EL element 102. In other words, a current Ids corresponding to the intersection 206 flows into the EL element.
Deterioration of the EL element 102 increases the resistance of the EL element 102. Then, the current-voltage curve (I-V curve) of the EL element 102 is changed from the curve 204 to a curve 205. In accordance with this change, the operation point of the EL element 102 is shifted from the intersection 206 to an intersection 207. In the saturation region 202, the dependence of Ids on Vds is small, and therefore the amount of current (Ids) flowing into the EL element 102 is not changed so much even if the I-V curve is changed from the curve 204 to the curve 205 due to the deterioration of the EL element by which the operation point is shifted from the intersection 206 to the intersection 207. On the other hand, in the linear region 203 in which the dependence of Ids on Vds is large, the amount of current (Ids) flowing into the EL element 102 is significantly changed when the EL element 102 deteriorates. Therefore, it is difficult to appreciate reduction in luminance of the EL element 102 when it is driven in the saturation region 202.
However, when the EL element 102 significantly deteriorates, the I-V curve of the EL element 102 is changed from the curve 204 to a curve 208, so that the operation point of the EL element 102 is shifted from the intersection 206 to an intersection 209. The intersection 206 is positioned in the saturation region 202 whereas the intersection 209 is positioned in the linear region 203, and therefore the amount of current which flows into the EL element 102 is largely changed to be reduced by ΔI in FIG. 2B. As a result, the luminance of the EL element is largely changed (FIG. 2B).
Patent Documents 1 and 2 are given as techniques for controlling the luminance of EL elements when the EL elements deteriorate.
In Patent Document 1, a display device has been disclosed in which two driving transistors having different characteristics are connected to an EL element and a means for selecting either one of the two driving transistors is provided so as to suppress the shift of the operation point of the EL element from the saturation region to the linear region due to deterioration of the EL element. For high-gray-level (high-luminance) display, a driving transistor having a capability of conducting high current is selected and a gate voltage of the driving transistor is decreased; and for low-gray-level (low-luminance) display, a driving transistor having a capability of conducting low current is selected.
Disclosed in Patent Document 2 is that a voltage between terminals of one EL element among a plurality of EL elements is detected and a voltage which is applied to a power source line is changed based on the detected voltage so that the luminances of the plurality of EL elements are controlled.